Circling the Drain

I heard this euphemism uttered by one of the doctors in the TV show Gray's Anatomy about the impending demise of a patient. Circle the Drain is also a song by Katy Perry and featured as a refrain in the lyrics of the song: 

Wanna be your lover, not your fucking motherCan't be your savior, I don't have the powerI'm not gonna stay and watch you circle the drainWatch you circle the drain, watch you circle the drain